The present invention relates in general to surround sound decoding and more particularly concerns novel apparatus and techniques for down-mixing a variable number of channels into a conventional stereophonic left and right channel pair, which when reproduced as a stereophonic pair, preserves the directional information of the originating left and right channel surround signals.
A typical surround sound signal includes at least left front, center front, right front, left rear, and right rear signals. A typical approach combines these signals into two signals that are typically decoded to recover a left front signal, a right front signal, a center signal and a monophonic rear signal representative of the sum of the original left rear and right rear signals.
It is an important object of the invention to provide improved apparatus and techniques for processing surround signals.
A feature of the invention resides in down-mixing a variable number of channels into a conventional stereophonic left and right channel pair which, when reproduced as a stereophonic pair, preserves the directional information of the originating left and right channel surround signals (also referred to as the left and right rear channel signals). Another feature resides in providing backward compatibility with existing matrix surround-sound decoder technologies such that the decoder technologies can substantially retrieve the encoded surround channel signals and their respective directional information.
The down-mixing apparatus typically includes a summer for summing the originating left and right channel surround signals into a single monophonic pair, level detectors for providing absolute value signals representative of the absolute values of the magnitudes of the originating left and right channel surround signals time averaged. A filter is coupled to the output of the summer and is constructed and arranged to filter the monophonic sum signal and typically having a single real pole in the transfer characteristic at a frequency of about 2 kHz and a single real zero at a frequency of about 1 kHz. There may be a pair of variable gain elements constructed and arranged to multiply the filtered sum signal by first and second time varying coefficient signals A1 and A2, respectively. The first coefficient signal is representative of the time averaged magnitude of the right channel surround signal divided by the sum of the time averaged magnitude of the left channel surround signal plus the time averaged magnitude of the right channel surround signal. The second coefficient is representative of the time averaged magnitude of the left channel surround signal divided by the sum of the time average magnitude of the left channel surround signal plus the time average magnitude of the right channel surround signal. The first and second multipliers provide first and second output product signals respectively HsA1, and HsA2. A subtractor subtracts the time varying product signals HsA1, and HsA2 from the unfiltered monophonic sum signal to produce the two encodable signals 1-HsA1 and 1-HsA2, respectively. These two encodable signals are then typically combined with a front center signal and at least respective ones of left and right signals to provide a stereo pair comprising left and right transmitted signals LT and RT, respectively, that may be coupled to left and right loudspeakers, respectively, to provide a transduced surround signal that reproduces components perceived by a listener as including directional information in the left and right surround signals.
An important aspect of the invention resides in providing time varying representations of the left and right surround signals, typically of magnitude that may be normalized magnitude relative to the sum of their magnitudes, and combining these time varying representations with at least left front and right front signals to provide the stereophonic pair.